Ten Hag have had experience in this league before so there's chances that he could do well, I'm not going to conclude until I see how Leverkusen would perform in their first 5-10 games until then Dortmund are the only team I feel can challenge Bayern Munich even though I doubt if any team would take away their dominance like Leverkusen did when Alonso was there. Well with the squad Xabi left for Ten Hag getting into the Champions league shouldn't be a big deal.
It was a very long time ago when Ten Hag managed Bayern Munich II. The current condition of Bundesliga is no longer the same as in the past. Ten Hag must have no much insight about the domestic competition in German recently. I'm sure he needs time to adapt with the local competition in German. Moreover, he is managing Leverkusen who has a big pressure. Ten Hag is expected to strengthen Leverkusen and stop the domination of Bayern Munich. Since some key players have left the team, I think we can't assume it is still the same as in the era of Xabi Alonso. You must be aware that Jonathan Tah, Jeremie Frimpong, and Florian Wirtz already moved to other teams. I personally assume this is a very big deal. Moreover Ten Hag still can't get the replacement with the same level.
Xabi left Ten Hag a well-established team and a squad with good chemistry. It would take a long time to make this squad compatible. In fact, the Spanish manager did a very good thing and made the team ready. For now, it will be enough for the Dutch manager to continue the system that works and make a few changes to it.
You can say this if there is no change in Leverkusen squad. Unfortunately, the players are no longer the same. Some of the key players have left the team. Ten Hag must rebuild Leverkusen squad, it is not an easy job. I think the most difficult thing is finding a new player to replace the role of Jeremie Frimpong and Florian Wirtz.